The present invention relates to power combiner networks and, more particularly, to the selection of multiple power levels using power combiners.
Power combiners are well-known devices that couple electromagnetic energy from multiple input ports to an output port in a prescribed manner. As is well-known, high power combiners are used in a number of application such as (i) combining two or more signals at the same or different frequencies for transmission by a common antenna; (ii) combining an analog signal and a digital signal for common antenna transmission, e.g., digital television and/or digital audio broadcast applications; and (iii) combining outputs of multiple power amplifiers.
The art is replete with power combiner arrangements for use, inter alia, in the above-described applications. For example, U.S. Pat. No. 4,315,222 issued to A. Saleh on Feb. 8, 1982, which is hereby incorporated by reference for all purposes, describes a power combiner arrangement for microwave power amplifiers which employs a series of sensing devices at the inputs to the combiner for identifying failed amplifiers at the inputs thereby improving the degradation performance of the microwave power amplifier. U.S. Pat. No. 4,697,160 issued to R. T. Clark on Sep. 29, 1987, which is hereby incorporated by reference for all purposes, describes a hybrid power combiner and controller for achieving power combination with improved finer amplitude control having reduced insertion loss. Further, U.S. Pat. No. 5,222,246 issued to H. J. Wolkstein on Jun. 22, 1993, which is hereby incorporated by reference for all purposes, describes a power amplifier arrangement employing a phase-sensitive power combiner for dividing a input signal into equal amplitude components for amplification purposes. As will be appreciated, the performance specifications of such power combiners continue to become more varied and stringent with the advent of new and/or expanded applications.
For example, in the United States AM/FM radio broadcast market, digital audio broadcast (xe2x80x9cDABxe2x80x9d) technology, e.g., so-called In-Band On-Channel (xe2x80x9cIBOCxe2x80x9d), is under consideration for widespread application. Digital audio broadcast applications are described, e.g., in Carl-Erik Sundberg, xe2x80x9cDigital Audio Broadcasting in the FM Bandxe2x80x9d, Proceedings of the IEEE Symposium on Industrial Electronics, Portugal, Jun. 1-11, 1997, and Carl-Erik Sundberg, xe2x80x9cDigital Audio Broadcasting: An Overview of Some Recent Activities in the U.S.xe2x80x9d, Proceedings of Norsig-97, Norwegian Signal Processing Symposium, Tromso, Norway, May 23-24, 1997, each of which are hereby are incorporated by reference for all purposes. Further, IBOC is described, e.g., in Carl-Erik Sundberg et al., xe2x80x9cTechnology Advances Enabling In-Band-On-Channel DSB Systemsxe2x80x9d, Proceedings of Broadcast Asia, June 1998, Suren Pai, xe2x80x9cIn-Band-On-Channel: The Choice of U.S. Broadcastersxe2x80x9d, Proceedings of Broadcast Asia, June 1998, and B. W. Kroeger et al., xe2x80x9cImproved IBOC DAB Technology for AM and FM Broadcastingxe2x80x9d, SBE Engineering Conference, pp. 1-10, 1996, each of which are hereby are incorporated by reference for all purposes. IBOC broadcasting systems utilize a digital overlay in the current FM analog broadcast band to deliver digital audio content. In accordance with IBOC, lower power digital signals, e.g., 20 to 30 dB below the analog signal level, are embedded as two sidebands on either side of the analog signal transmission within xc2x1200 kHz (off center frequency) as is required by current FCC regulations. As such, the digital sidebands are immediately adjacent to the analog band with virtually no significant separation between the frequencies of the analog and digital signals. Therefore, in order to achieve a degree of compatibility between the analog and digital signals, a sufficient isolation between the analog signal transmitter and digital signal transmitter must be achieved. In particular, a higher isolation is required from the analog transmitter to digital transmitter than from the digital transmitter to the analog transmitter because of the relatively large differential (e.g., 20 to 25 dB) in power levels between the two signals.
The challenge of achieving higher isolation, e.g., 60 to 80 dB, in an application such as IBOC, i.e., isolation between power sources where at least one source is much higher than the other, is to provide the requisite isolation with minimal degradation in insertion loss and group delay variation. As will be appreciated, depending upon the specific application the term xe2x80x9chigh powerxe2x80x9d will have different meanings. For example, in cellular applications, high power typically means 100 W or greater. Further, as will be appreciated, frequency proximity requirements also vary by application and impact such high power applications. More particularly, problems arise in high power combining when high isolation is required for signals having overlapping or nearly overlapping spectral occupancy characteristics. In cases where the signals are spectrally proximate but not overlapping, prior art high power combiners typically employ filtering in combination with power combining to increase isolation. However, the need for severe filter transitions, in the most proximal cases, often leads to undesirable distortions of the signals as they undergo the combining process. Furthermore, those signals to be combined that have overlapping spectral occupancies cannot benefit from these filtering schemes to increase isolation, but must rely solely upon inherent isolation of the core combiner.
Therefore, a need exists for a high power combiner with improved isolation between input ports for high power applications with minimal degradation in signal characteristics, e.g., insertion loss and/or group delay variation.
The present invention is directed to a high power combiner arrangement with improved isolation between input ports for high power applications. In particular, in accordance with the preferred embodiment of the invention, power combining logic is combined with a series of isolators such that at least one isolator is inserted between at least one power source, i.e., a signal source, and a corresponding input port to the power combining logic. The number and location of isolators inserted is determined as a function of the isolation requirements of the overall application. In accordance with the preferred embodiment, at least one isolator is a three port junction circulator device formed by a symmetrical junction transmission line coupled to a magnetically-biased ferrite material. Further, in accordance with preferred embodiments of the invention, the at least one circulator has at least one port terminated with a resistive matched load such that when one of the three ports of the circulator is terminated with the matched load, the circulator becomes an isolator which will isolate the incident and reflected signals at the remaining two ports.
Advantageously, in accordance with the invention, the degree of isolation achieved by the high power combiner is directly proportional to the number of isolators placed between each power source. Furthermore, the insertion of a number of high power circulators between each power source and the power combing logic facilitates the achievement of higher isolation between the power sources with limited degradation in signal characteristics.
In accordance with a further embodiment of the invention, the power combining logic is a hybrid coupler combined with a series of circulators such that at least one circulator is inserted between a power source and a corresponding input port to the hybrid coupler. As above, the number of circulators inserted is determined as a function of the isolation requirements of the overall application.